Systems and methods for measurement of optical wavefronts

ABSTRACT

An analytic tool for supporting alignment of an optical component in preparation for an interferometric test and performance of such a test. Apparatus and methods involve employment of the datum features on the optical component and/or metrology frame supporting such component. The metrology frame may include a secondary set of holograms (provided for use with a conventional system already employing a primary hologram that forms the testing optical wavefront). The conventional primary hologram is preferably substituted with a set of primary holograms (contained in the same, unitary or spatially-complementary housing sets) that perform different but complementary functions and that facilitate the alignment of the metrology frame with or without the tested optical component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This US Patent application claims priority from and benefit of the U.S.Provisional Patent Application No. 63/027,881 filed on May 20, 2020, thedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to optical metrology and, inparticular, to optical measurements of aspheric optical wavefronts (forexample, those representing and/or associated with optical componentsand systems having aspheric surfaces) with the use of opticalinterferometry.

RELATED ART

The use of wavefront sensing (in particular—optical interferometry and,specifically, phase-shifting interferometry) with computer-generatedhologram-based correction is recognized as a standard method formeasuring aspheric surface.

Phase-shift interferometry is an established method for measuring avariety of physical parameters that range from the shape of opticalcomponents to the density of gas in aerodynamic flow fields. Aninterferometric wavefront sensor, which employs phase-shiftinterferometry, typically includes a temporally-coherent light source(such as a laser, for example), the light output from which is spatiallysplit to define two optical wavefronts (a reference optical wavefrontand a test or object optical wavefront) propagating along differentoptical paths and later recombined after traversing different pathlengths. Upon the recombination, the relative phase difference betweenthese two wavefronts manifests as a two-dimensional intensity patternknown as an interferogram. Phase-shift interferometers typically have anelement in the path of the reference wavefront that is configured tointroduce multiple (usually three or more) known phase-steps orphase-shifts into the reference wavefront. By acquiring, with an opticaldetector, the irradiance patterns or interferograms corresponding toeach of such phase-shifts, the phase distribution of the objectwavefront can be quantitatively and rapidly calculated independentlyfrom the irradiance in the reference wavefront or the object wavefront.

An example 100, illustrating the process of testing of the workpiece 136with the use of the optical measurement system 140 (containing thecommercially-available phase-shifting interferometric system 110 that isjudiciously equipped for testing optical components) is schematicallyillustrated in FIG. 1. Here, the optical system of the interferometer110 (shown in this example as a Fizeau interferometer) spatially expandsa light output from a laser source (not shown) to a collimated beam oflight (not shown) and delivers it to the Fizeau transmission sphered114. The interchangeable (replaceable) Fizeau transmission sphere 114changes or modified the degree of collimation of this beam of light asdesired, and converges the substantially-spherical optical wavefront 118towards the focus 120. Upon passing the focal point 120, the sphericalwavefront starts spatially expanding to form the wavefront 124. At thesame time, the transmission sphere 114 partially reflects (not shown) aportion of the collimated beam incident onto the sphere 114 back to theinterferometer 110, to form a reference wavefront that is used in theinterferometric measurement.

The object optical wavefront 124 from the interferometer is transformedby the holographic component or system 128 (often—the one containing acomputer-generated hologram, or CGH, as shown) to the testing wavefront132 that closely (or substantially) matches the spatial profile of thesurface 136A of an optical component under test (or, interchangeably, aworkpiece under test or unit under test, UUT) 136. The light reflectedfrom or by this surface 136A (indicated with the double-headed arrow140) propagates back through the CGH 128 into the interferometer 110,where it is combined with the reference wavefront (formed by lightreflected into the interferometer by the reference surface). Fringes ofinterference (interferometric fringes, interferograms) formed as aresult of such optical interference are imaged onto an optoelectronicsensor or optical detector of the measurement system 140 (not shown).The phase of light in the measurement system 140 is discretely shiftedas the sequence of discrete images are acquired at the optical detector.This sequence of images is used to determine the shape (or spatialprofile) difference between the two interferometrically-combinedwavefronts (the reference wavefront and the wavefront formed by lightfrom the testing wavefront 132 that has been reflected by the surface136A). By carefully controlling and calibrating the reference wavefront,the shape departure from the ideal (or desired, or targeted) shape forthe surface 136A under test can then be determined as known in relatedart.

Often, a CGH 128 is manufactured onto (in cooperation with) a flat glasssubstrate or optical plate, which is mounted into a dedicated kinematicfixture 200 that includes a metallic frame 204 (see example of FIG. 2).The fixture 200 is typically equipped with steel spheres or balls suchthat the spatial position of the holographic pattern of the component128 is tightly controlled with respect to the steel balls. Such mountingfacilitates interchangeability of CGH 128 using kinematic seats todefine the positions of the balls. A dedicated standard alignment CGH128 can be used for optical alignment. Once the alignment is complete,this alignment CGH may be removed from the kinematic fixture 200, andreplaced with a different CGH, while the kinematics of the fixture 200while the accuracy of the positioning of the new holographic patternwith respect to the balls is maintained within the acceptable window orrange of deviation.

The challenges and recognized limitations of this currently-accepted andused in related art methodology of measurement manifest in how preciselythe target aspheric surface of a given workpiece under test can bespatially positioned with respect to the testing (measurement) wavefront132. The precision of such positioning, as will be readily understood bya skilled artisan, directly affects the spatial profile of themeasurement wavefront 132 at the moment of incidence onto the workpieceor the unit under test (UUT). (For example, optical wavefronts that arespatially expending will have a radius of curvature that increases asthe wavefront propagates, so if the surface under test 136A—in theexample of FIG. 1—is positioned too far away from the generator of thetesting wavefront 132—here, the CGH 128—the wavefront 132 will have alarger radius of curvature at the moment of incidence on the surface136A. A workpiece 136 that ensures a null measurement in this situationwill necessarily have a radius of curvature of the surface 136A that istoo large for practical use. This situation persists for aspherical aswell as spherical wavefronts.) The challenges originate from theadoption of a variety of rather complex aspheric surfaces that are nowrequired in production of optical systems that must achieve very highquality of optical imaging. Additional degrees of freedom beyond thespacing must be controlled in the test configuration for the accuratemeasurement of aspheric surfaces.

As a person of skill in the art will readily appreciate, the simplestaspheric surfaces are shapes defined by conic sections of revolution,such as paraboloid, ellipsoid, and hyperboloid. For increasedperformance in the optical system, additional polynomial terms are oftenadded to the function of revolution. Often, an optical system will useonly an off-axis portion of such an axisymmetric shape such that it isimpractical to make or measure the full parent. A general class ofaspheric surfaces are now being used that are called “freeformsurfaces,” which include can include nearly any smooth shape defined bynumerical functions or even defined as grids of points. While somemethods of controlling the configuration of the test system such as thesystem 100 have been implemented, there remains an unsatisfied need insolutions for quick and precise alignment of the UUTs with asphericalsurfaces that lend themselves to measurement en masse, with highthroughput.

SUMMARY

Embodiments of the invention provide an apparatus for measuring anoptical wavefront representing an optical workpiece under test, thatfacilitates the alignment of the workpiece with respect to all sixdegrees of freedom. Such apparatus includes a wavefront sensor and firstand second repositionable systems located across the axis of theapparatus and one after another with respect to an output from thewavefront sensor. The first repositionable system contain a firstalignment hologram, a first measurement hologram, and a second alignmenthologram such as to form a primary alignment wavefront (by reflecting afirst portion of the light output from an optical wavefront deliveredfrom the wavefront sensor and incident onto the first repositionableoptical system) and to transmit a second portion of the light output.The second repositionable system contains an alignment referencecomponent and at least one optical workpiece held in a fixed positionand a fixed orientation with respect to the alignment referencecomponent. The second repositionable system is disposed to reflect,respectively, a first optical wavefront from the second portion of thelight output (that has been transmitted through the first system) and asecond optical wavefront from the same second portion through the firstrepositionable system towards the wavefront sensor. The apparatusadditionally includes a positioner configured to simultaneously changei) at least one of a position and an orientation of the alignmentreference component and ii) at least one of a position and anorientation of the at least one optical workpiece. In a specific case,the apparatus may be complemented with a mounting substrate installedacross the axis and separated from the wavefront sensor by the firstrepositionable system. When so structured, the configuration of theapparatus satisfied one or more of the following conditions: a) thealignment reference component includes at least one of a reflectivehologram attached to the mounting substrate, a reference surface of anoptical element holder attached to the mounting substrate, and areference surface of an optical element in the optical element holderattached to the mounting substrate; b) the at least one opticalworkpiece is affixed to the mounting substrate; c) the at least oneoptical workpiece includes a plurality of optical workpieces, eachworkpiece held in a fixed position and orientation with respect to thealignment reference component; d) the at least one optical workpiece andthe alignment reference component are removably affixed in the mountingsubstrate; and e) the positioner is operably cooperated with themounting substrate to change the at least one of position andorientation of the alignment reference component and the at least oneoptical workpiece simultaneously while changing at least one of positionand orientation of the mounting substrate. Alternatively or in addition,the apparatus may be configured such that at least one optical workpieceis held in an opening defined through the mounting substrate, and/orfurther include a reference reflector positioned to receive light fromthe second wavefront through the at least one optical workpiece andreflect said light back onto itself, through the at least one opticalworkpiece, the first repositionable system, and into the wavefrontsensor.

Related embodiments also provide an apparatus (for measuring an opticalwavefront representing an optical workpiece) that includes a wavefrontsensor; a first repositionable system that contains a first alignmenthologram, a first measurement hologram, and a second alignment hologramand that is disposed across the axis of the apparatus such as to form aprimary alignment wavefront by reflecting a first portion of lightoutput from an optical wavefront delivered from the wavefront sensor andincident onto the first alignment hologram, and to transmit a secondportion of said light output through the second alignment hologram. Theapparatus additionally includes a second repositionable system thatcontains a reflective hologram (disposed across the axis to reflectlight from the second portion through the first optical system to form asecondary alignment wavefront propagating through the firstrepositionable system towards the wavefront sensor) and tangiblefiduciary references outside of the reflective hologram (here, suchfiduciary references are structured to spatially align the workpiecewith respect to the second repositionable system). In a specific case,the wavefront sensor may include an optical interferometer and/or thefirst repositionable system may be disposed at a location at which saidoptical wavefront delivered from the wavefront sensor is eitherspatially-diverging or spatially converging and/or the reflectivehologram may be configured to include at least one reflectivediffraction pattern designed to form the secondary alignment wavefrontthat represents at least one of the spatial tilt, azimuthal angulardeviation, transverse shift, and longitudinal shift of the secondrepositionable system with respect to the axis. Additionally or in thealternative—and in any implementation of the apparatus—the firstmeasurement hologram may be made a transmissive hologram configured totransform the incident optical wavefront into an aspheric opticalwavefront and/or the first repositionable system may include a firststand-alone repositionable component containing the first alignmenthologram and a second stand-alone repositionable component containingthe first measurement hologram (here, the first measurement hologram isconfigured to transform the incident optical wavefront into an asphericoptical wavefront).

Embodiments additionally provide a method for measuring an opticalwavefront characterizing an optical workpiece. Such method includes astep of determining a misalignment of a first system with respect anaxis with the use of a substantially-spherical optical wavefrontincident thereon from a wavefront sensor (here, the first systemcontains at least a first alignment hologram, a first measurementhologram, and a second alignment hologram) and a step of redirectingfirst and second optical wavefronts, formed by transmitting thesubstantially-spherical optical wavefront respectively through the firstand second alignment holograms, towards a second system containing amounting substrate and an alignment reference component disposed in afirst opening of the mounting substrate. The method additionally includesteps of forming reflected first and second optical wavefronts (byrespectively interacting the first and second optical wavefronts withthe optical workpiece fixatedly positioned in a second opening of themounting substrate and the alignment reference component) andpropagating the reflected first and second optical wavefronts throughthe first system towards the wavefront sensor. The method furtherinvolves spatially aligning the alignment reference component affixed atthe mounting substrate to eliminate at least one of the spatial tilt,azimuthal angular deviation, transverse shift, and longitudinal shift ofthe alignment reference component with respect to the axis (by changingat least one of position and orientation of the mounting substrate withrespect to the axis based on a measurement of light from the reflectedsecond optical wavefront acquired at the wavefront sensor) anddetermining an error in the reflected first optical wavefront based on ameasurement of light from the first reflected optical wavefront acquiredat the wavefront sensor. Alternatively or in addition, the method mayinclude a step of substantial nullification of the misalignment of thefirst optical system (by partially reflecting thesubstantially-spherical optical wavefront at the first alignmenthologram of the first system towards the wavefront sensor to form areflected wavefront) and/or a step of at least reducing the misalignmentof the first system (by minimizing a figure of merit determined based ona measurement of a phase of the reflected wavefront at the wavefrontsensor) and/or a step of spatially fixating a component of the firstoptical system with respect to the axis after said figured of merit hasbeen minimized. (In the latter case, the process of determining theerror in the reflected first optical wavefront may be carried out afterthe step of at least reducing the misalignment of the first system hasbeen accomplished.) Alternatively or I addition, the method may beconfigured such that—when the wavefront sensor is an opticalinterferometer—at least one of the following conditions is satisfied: i)the process of determining the error in the reflected first opticalwavefront includes determining a difference between a spatial profile ofthe reflected first optical wavefront that has transmitted through thefirst system towards the interferometer and a spatial profile of areference optical wavefront generated internally to the interferometer;ii) the process of spatially aligning the alignment reference componentincludes forming alignment optical interference fringes by opticallyinterfering (at an output of the optical interferometer) the reflectedsecond optical wavefront that has transmitted through the first systemand the reference optical wavefront; and iii) the process of at leastreducing the misalignment includes reorienting the component of thefirst system to substantially eliminate at least one of the spatialtilt, azimuthal angular deviation, transverse shift, and longitudinalshift of such component with respect to the axis based on transformationof the optical interference fringes formed at the output of the opticalinterferometer. In at least one implementation of the method, the firstsystem includes first and second spatially-distinct and separable fromone another components (here, the first component contains one or moreof the first alignment hologram, the second alignment hologram, and thefirst measurement hologram while the second component contains remainingof the first alignment hologram, the second alignment hologram, and thefirst measurement hologram). In substantially any implementation of themethod, the steps of redirecting and spatially aligning may be—andpreferably are—carried out without a relative movement between theworkpiece and the mounting substrate and without a relative movementbetween the alignment reference component and the mounting substrate. Insubstantially any implementation of the method, the step of redirectingthe second optical wavefront by interacting said second opticalwavefront with the alignment reference component may include at leastone of (a) reflecting the second optical wavefront by a reflectivehologram contained in the second optical system; (b) reflecting thesecond optical wavefront at a reference surface of an optical elementholder held in a respectively-corresponding opening of the mountingsubstrate; and (c) reflecting the second optical wavefront at areference surface of an optical element mounted in the optical elementholder held in the respectively-corresponding opening of the mountingsubstrate. In substantially any implementation of the method, theprocess of redirecting first and second optical wavefronts may includeredirecting a plurality of measurement optical wavefronts (each of whichis formed by transmitting the substantially-spherical optical wavefrontthrough a plurality of the measurement holograms of the first system)and forming simultaneously a plurality of reflected measurement opticalwavefronts (by respectively interacting each of the plurality ofmeasurement optical wavefronts with a corresponding workpiece from aplurality of workpieces disposed in respectively-corresponding openingsin the mounting substrate).

Embodiments of the invention additionally provide a related method formeasuring an optical wavefront characterizing an optical workpiece witha wavefront sensor. Such method includes determining a misalignment of afirst system with respect to an axis with the use of asubstantially-spherical optical wavefront incident thereon from thewavefront sensor (here, the first system contains a first alignmenthologram, a first measurement hologram, and a second alignmenthologram); redirecting a first optical wavefront, formed by transmittingthe substantially-spherical optical wavefront through the secondalignment hologram, towards a second system containing at least onereflective hologram; forming a reflected first optical wavefront byinteracting the first optical wavefront with the at least one reflectivehologram; and aligning the second system with respect to the axis byreducing at least one of spatial tilt, azimuthal angular deviation,transverse shift, and longitudinal shift of the at least one reflectivehologram by measuring an error of the reflected first optical wavefront(that has transmitted through the first optical system) with thewavefront sensor. The method further includes the steps of juxtaposingand affixing the optical workpiece with the second system in referenceto visually-perceivable fiducial features of the second system;redirecting a second optical wavefront (formed by transmitting thesubstantially-spherical optical wavefront through the first alignmenthologram) towards the second system to form a reflected second opticalwavefront by interacting the second optical wavefront with theworkpiece; and determining an error in the reflected second opticalwavefront based on a measurement of light from the reflected secondoptical wavefront acquired at the wavefront sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description of Specific Embodiments in conjunctionwith the Drawings, of which:

FIG. 1 schematically illustrates conventionally-configured methodologyfor assessing an optical wavefront representing an optical unit undertest (a workpiece) with the use of a measurement system including andinterferometric system and a holographic element (here, a CGH).

FIG. 2 shows a kinematic mount for repositionably-holding the CGH in aconventional measurement illustrated in FIG. 1.

FIG. 3 schematically illustrates schematically the measurement systemconfigured according to the idea of the invention.

FIGS. 4A, 4B, 4C, and 4D provide an illustration to methodology ofalignment of optical components of an embodiment of the system of theinvention and the following measurement of the UUT or workpiece with theuse of such system.

FIGS. 5A, 5B provide illustration to methodology of alignment of opticalcomponents of a related embodiment of the system of the invention andthe following measurement of the UUT with the use of such system.

FIGS. 6A, 6B, 6C provide illustration to methodology of alignment ofoptical components of yet another related embodiment of the system ofthe invention and the following measurement of the UUT with the use ofsuch system.

FIGS. 7A, 7B, and 7C provide schematic illustrations to the structureand operation of a related embodiment of the invention.

FIG. 8 depicts the measurement of a multi-element UUT in transmission.

FIGS. 9A, 9B schematically illustrate specific geometry of the secondoptical system of the embodiment of the invention configured fordetermination of the axial displacement of the metrology frame of theinvention.

Generally, the sizes and relative scales of elements in Drawings may beset to be different from actual ones to appropriately facilitatesimplicity, clarity, and understanding of the Drawings. For the samereason, not all elements present in one Drawing may necessarily be shownin another.

DETAILED DESCRIPTION

The fundamental shortcoming persisting in related art comes from thedemand for higher performance optical systems that take advantage of theability to produce more complex optical elements, yet the existingmethods for optical measurement of these elements (including theiralignment in the measurement optical system(s)) suffer from eitheraccuracy limitations (the alignment for optical testing requires sixdegrees of freedom) and/or are not fast enough to support volumeproduction.

This problem is addressed by embodiments of the present invention thatprovide, in one example—in reference to the schematic of FIG. 3—anoptical measurement system 300 containing a wavefront sensor 310 (in aspecific case—the interferometer-based optical wavefront sensor), afirst system containing a first optical system that includes a primaryhologram 328 (which, generally, corresponds to the element 128 of theconventionally-structure measurement system), and a metrology frame 350that may include or be cooperated with or carry a second optical system(the combination of the metrology frame with its contents is referredbelow as a second system). The combination of the contraptions 310 and328 is denoted as 340—and, generally, such combination corresponds tothe system 140 of FIG. 1. The axis of the system (and of the primaryhologram 328, when the primary hologram is properly aligned) is denotedas 320, as is associated with the z-axis of the local system ofcoordinates. According to the idea of the invention, the metrology frame(with or without the second optical system) is judiciously configuredfor alignment and measurement of the workpiece under test (once suchworkpiece is juxtaposed with the metrology frame) with respect to thetesting wavefront emanating from the first optical system 328. For thepurposes of this disclosure and the appended claims, the term wavefrontis generally defined according to the common understanding of this termin related art—as a surface that is transverse to the wavevector of amonochromatic wave and in which such wave maintains the same, constantphase.

The metrology frame 350 may contain—depending on the specificimplementation—the second optical system that includes an auxiliary orsecondary CGH (disposed to reflect light back to the primary CGH 328)and/or a set of reference surfaces or tangible features (interchangeablyreferred to as datum surfaces or features) that are accuratelymanufactured, both in their shape and position. According to the idea ofthe invention, measurements performed with the use of the second opticalsystem of the metrology frame 350 are used to define the position of themetrology frame with respect to the optical wavefront-sensor-basedmeasurement system 340. In some cases the unit under test UUT (notshown) is an aspheric surface which affects the shape of the wavefrontupon reflection. Alternatively the UUT could include a system orcombination of multiple optical elements that affect the wavefront uponthe transmission of such wavefront through this combination and/or uponreflecting such wavefront off of the combination while relativepositions of the optical elements in such combination may be controlledwith the use of employed mounting hardware.

Preferably, the wavefront sensor and the first and second systems areoperably cooperated with the programmable computer processor (indicatedin FIG. 3 as PC) that is cooperated with the tangible non-transitorystorage medium carrying thereon program code that, when loaded onto theprocessor, governs at least the acquisition of optical data from thewavefront sensor and/or the relocation/repositioning/reorientation of atleast one of the first and second systems.

Datum or reference or fiduciary features on the metrology frame 350(such a visually-perceivable markings, surface relief or structures, orother tangible indicia) are used to judiciously define the requiredposition of the workpiece in and/or in reference to the metrology frame.As will be understood from the following disclosure, one implementationthe related measurement procedure of the workpiece UUT with the use ofthe system 300 may generally involve:

-   -   A step of calibrating the measurement system 340. This        calibration procedure typically addresses the calibration of the        wavefront sensor 310 (unless the wavefront sensor 310 has been        already pre-calibrated) and at least the determination or        measurement of misalignment of the primary CGH 328 with respect        to the wavefront sensor 310. (The physical correction of such        misalignment, resulting from mutual repositioning and/or        reorientation of the CGH 328 at this step is considered to be        optional);    -   A step of alignment of the metrology frame 350 (which has been        already structured to have datum features and/or the secondary        CGH to be used for alignment of the UUT or optic under        test)—with or without contents of the metrology frame—with        respect to the primary CGH 328 using the predetermined alignment        patterns;    -   Juxtaposing the optical workpiece or UUT (not expressly shown in        FIG. 3) with the metrology frame 350 and aligning it with        respect to the metrology frame 350 using the available datum        references for definition of all 6 degrees of freedom (DoFs);        and    -   Measuring the so-positioned/aligned optical workpiece with the        use of optical wavefront(s) delivered to the metrology frame 350        from the sub-system 340.

After the measurement of a given optical workpiece has beenaccomplished, and in order to measure a new optic with identicalprescription (which term conventionally denotes the definition of theideal shape for an asphere or a parameter for a set of optical elements,as a skilled artisan knows), the first optical workpiece is simplyremoved from the metrology frame 350 and the new optic is inserted intothe same metrology frame using the same datum references. To measure anew optical workpiece with new prescription, however, the primary CGH328 must understandably be replaced (to generate a different testingwavefront corresponding to the new optical workpiece). Even in thiscase, however, if the required mounting features of the new opticalworkpiece are identical, then it is possible to use the same metrologyframe configured according to the idea of the invention.

Advantages provided by embodiments of the current invention becomeclearer once the limitations of methodologies of measuring complexaspheric surfaces currently accepted and used by related art areconsidered. Currently used methods of measuring complex asphericsurfaces fall within several classes:

-   -   Precision profiling. Here, highly accurate machines are        available that scan touch or optical probes over the surface to        measure substantially any shape. If cost is not an issue,        equipment is available that measures both optical surfaces and        mounting interfaces to the precision required. The limitation        here comes from the cost of the machines and the throughput. It        can take many minutes to provide an accurate scan of even a        small part and the machines are too expensive to achieve high        rates with parallel measurement lines.    -   Optical scanning. Several technologies scan patches of        interferometric measurements over the surface and combine them        with software to determine the shape of the full surface. These        machines have two limitations, any particular machine will be        limited in the class of aspherics that it can measure—typically        only axisymmetric parts. Since this methodology relies on        scanning, the measurements and the data reduction are time        consuming, to say nothing about difficulties for non-expert        users to set up the required measurements such that they are        assured accuracy.    -   Standard interferometry. Commercial systems are available that        measure surfaces that have only a small departure from flat or        spherical. These machines provide quick snapshot measurements        that can achieve nm accuracy. The limitation of interferometry        comes from the available dynamic range. Without the addition of        additional optics or CGHs, the interferometer is limited to        measuring a surface that has only a few micron departure from a        reference spherical surface.    -   Interferometry with null correctors. The addition of a null        corrector (another optical system that combines with the        interferometer to give a null measurement for a particular        shape) allows for the measurement of an aspheric surface with a        standard interferometer. Classically, the null corrector        comprises a set of lenses or mirrors, but nearly all modern null        correctors utilize computer generated holograms. A new CGH must        be designed and manufactured for each new aspheric surface. The        CGHs have line patterns written onto them that use diffraction        to change the wavefront shape from the spherical interferometer        to the aspheric surface. CGHs written using modern lithographic        techniques allow surface measurements of nearly any aspherical        surface to nm accuracy.

Example 1

In further reference to FIG. 3, FIGS. 4A, 4B, 4C provide a schematicillustration to an embodiment of the invention, in which the metrologyframe 350 incorporates or contains or carries an auxiliary (or second,or metrology reference) optical system 418. The second optical system418 includes at least one secondary or auxiliary holographic layer (inone specific case—at least one secondary hologram configured as anoptical reflector).

Here, the primary optical system 428 (that contains an CGH with aholographically-defined reflective pattern and that corresponds to theelement 128 of FIG. 1 and to the contraption 328 of FIG. 3) iskinematically mounted, as known in related art (see FIG. 2). However,according to the idea of the invention the system 428 can be modified ascompared with the conventional component 128. In particular, the system428 generally can be configured—as discussed in detail below—as aunitary, single-piece system or as a combination of at least twostand-alone sub-systems. In the latter case (which is in advantageouscontradistinction with the way the element 128 is configuredconventionally in related art), one of the at least two such stand-alonesub-systems includes a holographically-defined reflective pattern usedfor alignment of the system 428 with respect to the WS 410 (not shown)while another is used for measurement of an optical wavefrontrepresenting a workpiece or UUT disposed in precise spatial coordinationwith the metrology frame 350 using the datum features of the frame 350.(Examples of such datum features are provided by markings, surfacerelief, edges, or other fiducial elements.) In operation, the primaryoptical system 428 (whether it contains only one, single-piece system ora multi-piece system) of an embodiment of the invention in used in placeof the component 328 of FIG. 3 or in place of theconventionally-employed component 128 of the system 100 of FIG. 1.

Pre-alignment or determination of misalignment of the primary hologramwith respect to the chosen axis. In reference to FIG. 4A—and before theprocess of the measurement of the workpiece is carried out—the primaryoptical system 428 is preferably appropriately aligned with respect tothe WS 410 and/or axis 320—or at least its relative misalignment withrespect to the WS 410 and/or axis 320 is determined.

Alignment of the metrology frame with respect to the chosen axis.According to the idea of the invention, and referring now to FIG. 4B,the second optical system 418 is now added to the optical train to beseparated from the WS 410 by the system 428 such as to intersect aportion of the optical wavefront 424 that is transmitted through thefirst system 428. The second optical system 418 is housed or held in orcarried by a dedicated harness system (such as an appropriate mountinghardware cell of the metrology frame 350) that includes tangiblereference/datum features with respect to which position and/ororientation of the optical system 418 can be precisely ensured and/ordetermined). The second optical system 418 includes a plurality ofreflective metrology reference holograms, 444. Such reflective metrologyreference metrology holograms (not shown; representing diffractivepatterns built into the body of the system 418) are configured toredirect light from test or measurement wavefront(s) arriving onto thesystem 418 through and from the system 428 by diffracting these arrivingwavefront(s) in reflection and through the first optical system 428 andtowards the WS 410. Once diffracted back to the system 428, the test ormeasurement optical wavefronts can be referred to as return orredirected test wavefronts 440, and, upon reaching the WS of the overallsystem, be measured to acquire and/or determine their spatial profileswith respect to the pre-determined reference of the overall measurementsystem. A skilled artisan will readily appreciate that in one specificcase when the WS 410 is configured as an optical interferometer, thereturn optical wavefronts 440 are optically-interfered with a referenceoptical wavefront (formed internally to the interferometer, for examplewith the use of a Fizeau transmission sphere) and the difference betweena given returned wavefront and the reference wavefront is thendetermined with the use of the measurement involving assessment of theresulting interferogram.

Accordingly, the alignment of the optical system 418 with respect to theoptical system 428 (the misalignment of which with respect to the WSand/or the reference axis 320 has been already determined and preferablycorrected) can be performed.

The metrology reference holograms 444 encoded in the optical system 418are judiciously structured to provide for alignment of the system 418 bymeasuring all 6 degrees of freedom. The design of such metrologyreference CGHs involves a tradeoff of the size of the holographicpatches, the measurement precision for each DoF, and the dynamic rangefor each measurement. (Once the second optical system 418 containingmultiple reference holograms 444, and with it—the metrology frame350—has been aligned with respect to the axis 320, the next step of themeasurement of the wavefront carrying the information about theworkpiece under test can be accomplished, as discussed below inreference to FIG. 4D.)

Referring again to FIG. 4B and considering the illustration ofrepresentation of various optical aberrations with the use of Zernikepolynomials in FIG. 4C, in one specific implementation, the metrologyframe 350 may be structured to include the second optical system 418that contains or carries an auxiliary CGH 444 configured as adiffractive element simulating a spherical (for example, concave)reflector. In this case, the use of the returned-by-the-system 418wavefront 440 improves the quality of alignment of the system 418 (ascompared with the alignment based on reflection off of the tooling ball,known and currently utilized in related art). For reflections from asingle spherical surface, it is impossible to discern tilt of thesurface from decenter. (Due to the symmetry of the spherical ball, thetilt and decenter have exactly the same effect on the wavefrontreflected from the tooling ball.)

However, by implementing the idea of the invention the degeneracy of theconventional tooling sphere or tooling ball can be addressed byoptionally configuring the auxiliary CGH 444 in the optical system 418to produce a wavefront with a higher-order spatial profile; see FIG. 4C.For example, spherical aberration (or Z(4,0), in Zernike terms) can beapplied to a flat reflective surface, along with the matching wavefrontfrom the measurement hologram. The interferometer 410 will measure anull (ideal wavefront) for the case of alignment. If the CGH of theoptical system 418 is tilted (for example, as a result of angularmisalignment of the metrology frame 350 with respect to the axis 320),then the interferometer 410 will measure the tilt. If the CGH-basedreflector 444 of the secondary system 418 is translated or shiftedlaterally with respect to the direction of light propagation=such asaxis 320 (as a result of the respective translation of the metrologyframe 350), then the interferometer will measure coma (or Z(3,1) andZ(1,3), in Zernike terms). The amount of coma is proportional to thelateral shift.

Indeed, the interferometer measures the difference between the spatialprofile of the optical wavefronts reflected by the CGH of the system 418and that of the reference optical wavefront generated at theinterferometer. The low order components can be decomposed (for the caseof circular data sets) into Zernike polynomials. Even Zernikepolynomials are defined as

Z _(n) ^(m)(p,φ)=R _(n) ^(m)(ρ)cos(mφ)

and odd Zernike polynomials are defined as

Z _(n) ^(−m)(ρ,φ)=R _(n) ^(m)(ρ)sin(mφ),

where m and n are non-negative integers. ρ is the normalized radialdistance and φ is the azimuthal angle in radians.

${R_{n}^{m}(\rho)} = {\sum\limits_{k = 0}^{{({n - m})}/2}{\frac{\left( {- 1} \right)^{k}{\left( {n - k} \right)!}}{{k!}{\left( {{\left( {n + m} \right)/2} - k} \right)!}\left( {{\left( {n - m} \right)/2} - k} \right)}\rho^{n - {2k}}}}$

While the longitudinal shift (axial displacement of the CGH 444 of thesystem 418, together with the metrology frame 350) along the directionof the light propagation (axis 320) will have no effect, such axialdisplacement can be determined using specific geometry of the opticalsystem 418 as illustrated in the example of FIGS. 9A, 9B (in a spatiallydiverging wavefront delivered from the optical system 428 to the opticalsystem 418). In FIGS. 9A, 9B only the optical system 418 of themetrology frame is illustrated for simplicity. This geometry couples,links the axial motion of the system 418 into relative lateralmeasurements of first and second specifically-designed metrologyreference holograms 444—as shown, the reflective CGHs 944A, 944B of thesystem 418 that are spatially separated from one another and theposition and orientation of which does not change with respect to theoptical system 418. (In one embodiment, the optical system 418 isconfigured as a glass plate carrying the reflective CGHs 944A, 944B.)

The shift of each reflective CGH 944A, 944B is a function of theoff-axis distance, angularly represented as θ. The lateral shift of thereflection CGH 944A, 944B with respect to the wavefront comes fromsimple geometry, and can be calculated as Δx=Δz tan(0), Δz being anaxial (longitudinal) shift of the system 418. Understandably, theaverage lateral Δx motion as measured provides the net lateral motion orshift and the difference between the two is used to calculate the axialmotion or shift of the metrology frame 350

Once the secondary optical system 418 containing multiple reflectiveholograms 444 (among which there may be CGHs 944A, 944B) and with it—themetrology frame 350—has been aligned in reference to the axis 320, themeasurement of the wavefront representing the UUT 480 can be carriedout. Referring now to FIG. 4D, at this step the UUT or workpiece 480 ispositioned in/located in/juxtaposed with the (now aligned in referenceto axis 320) metrology frame 350 in the predetermined orientation withrespect to the fiducial features of the frame 350. Appropriatetransmissive holograms (or hologram patches) of the optical system 428are then used to form the testing or measurement optical wavefront 432.(As was already alluded to during the discussion of the system 100, thetesting optical wavefront preferably has spatial profile substantiallyapproximating the expected spatial profile of the surface of the UUT480. Therefore, the corresponding CGH patches of the system 428 shouldbe design to produce such a wavefront.) The wavefront 432 is thendelivered to the UUT affixed in the frame 350 in reference to itsfiduciary features and, upon reflection from the UUT 480, is deliveredin transmission through the optical system 428 to the interferometer forthe determination of a deviation (or difference) between the wavefront432 reflected by the UUT and the reference wavefront (that is formedinternally to the interferometer).

Example 2

FIGS. 5A and 5B illustrate a related embodiment, in which the metrologyframe 350 incorporates an optical component mount or holder 508judiciously configured to incorporate specific fiduciary or datumstructures and/or features 512, 514. In one example, such datumstructure(s) include(s) a conical surface 512 precisely machined (forexample, via diamond turning) at the internal side of the wall 516 (ofthe mount 508) that is dimensioned to accommodate the UUT to be tested,and/or a flat surface 514. As shown schematically in FIG. 5A, the mount508 is accurately aligned with respect to the first optical system 428before the optic (UUT) under test 480 is installed in the mount 508 inreference to and with the use of the same datum features 512. Thisalignment is performed with the use of the alignment opticalwavefront(s) 518, 520 formed at the optical system 428 as a result ofinteraction of the incident optical wavefront 424 with the appropriatelyconfigured transmissive alignment holograms (or hologram patches; notexpressly shown) of the system 428. As shown here, the fiduciarysurfaces 512, 514 are used to substantially retro-reflect the respectiveincident wavefronts 520, 518 back to the WS 410 through the opticalsystem 428.

In the illustration of FIG. 5B, the UUT 480 to be tested is structuredto be equipped with the datum feature of its own—in this example shownas the conical edge surface 522 that is substantially congruent with thedatum reference surface 512 of the mount 508—thereby facilitating andguaranteeing the correct installation of the UUT 480 in the mount 508and automatic alignment with respect to the measurement opticalwavefront 530 that arrives from the optical system 428 as a result ofdiffraction of the incident wavefront 424 at the appropriatetransmissive measurement hologram patches of the system 428. Once theworkpiece or UUT 480 is juxtaposed with the mount 508 that is carried bythe pre-aligned (at the step of FIG. 5A) metrology frame 350, theretro-reflection of the testing wavefronts 530 (produced by yet anotherset of holographic patches of the system 428; not shown) through thesystem 428 towards the WS 410 gives rise to interferograms representingthe deviation of the light-reflecting surface of the workpiece 480 fromthe testing wavefront(s) 530.

Example 3

In yet another related embodiment- and in reference to FIGS. 6A, 6B,6C—the procedure of alignment of the metrology frame 350 in reference tothe optical system 428 is accomplished with the use of a surrogateoptical component 614. The surrogate component 614 is judiciouslystructured to possess not only datum (reference) feature(s) 612 that aresubstantially identical to and/or congruent with the datum features (see512) of the mount 508 and of the UUT to be tested, and also has apre-determined structural features (such as a reflective surface 626)dimensioned to be interrogated with the use of the optical alignmentwavefront 630 that are generated at the appropriately configuredhologram patches in the first optical system 428 and that are configuredfor use in the alignment portion of the procedure. The surrogate optic614 is held in the mount 508 with the use of the mutually-matchingfiduciary features 612, 512 (FIG. 6B) to form a combination 618. Themisalignment of the combination 618 with respect to the system 428 isassessed with the use of the optical wavefront 630 returned inreflection from the surface 626 and through the system 428 into theinterferometer 410 (not shown). The metrology frame 350 (and with it,the mount-surrogate-optic combination 618) is then aligned, 520.Thereafter, the surrogate optic 614 is simply replaced with the UUT 480using the identical surface to define its position in the mount 508, forinterferometric measurement with the use of appropriate measurementwavefronts 530 generated at the corresponding transmissive holograms ofthe system 428.

The CGH 428 can generally be fabricated from fused silica glasssubstrate with an appropriate line pattern defined according to thesimulations of diffraction of light and etched into the substrate todefine required holographic pattern(s). Some of these patterns areconfigured to form alignment wavefronts 630 (that are designed toreflect off the surfaces manufactured onto the surrogate 614) from theincident wavefront 424. The simplest of the surfaces 626 are sphericalor flat, but in some implementations the use of more complex shapes suchas those described above may prove to be beneficial.

Notably, implementations of the system of the invention discussed aboveare not mutually exclusive. For example, a second optical system held inor being part of the metrology frame and used in addition to the firstsystem 428 generally may include at least one of the systems 418, 508,614, 618 that is/are used for the corresponding alignment and metrologysteps.

Example 4

Mass production of high-performance optical systems (such as theadvanced cameras in current generation cell phones) is enabled by glassand plastic molding technologies. Current measurement methods are provento be too slow and expensive to measure each part by itself, so amanufacturers must control the process very tightly to ensure thatdefective parts are not built up into the assembly. Understandably,there exists an economic premium for measuring a higher fraction of theoptical elements and subsystems.

A related embodiment of the invention is judiciously configured topermit the user to carry out simultaneous (a one-step) alignment ofmultiple UUTs (or workpieces) as well as simultaneous measurement of thewavefronts representing these UUTs regardless of whether these UUTs areidentical or different from one another. This approach is now describedin reference to FIGS. 7A, 7B, and 7C.

Here, according to the idea of the invention, the metrology-frame-basedmeasurement methodologies described above are used for high-volumemeasurement of small optical elements, surfaces, and/or systems.Specifically, multiple—and differing from one another—optical componentsor systems to be measured can be placed using their fiduciary or datumsurfaces to a custom mount or carrier possessing fiduciary interfacesmatching and mating with those of multiple parts at the same time. Highmeasurement throughput for the overall system is envisioned to beenabled using several carriers, so while one is being loaded, one can bemeasured, and another one can be unloaded. High accuracy is achievedwith the intrinsic accuracy provided by the wavefront-based sendingcoupled with active alignment using fiduciary elements (such as, forexample, a CGH).

The described approach turns on the realization that standardization forthe mounting interface for multiple UUT components can be achieved withthe use of the above-described metrology platform that supports suchmultiple UUT components with an ensemble of precision “seats” matchingthe final interfaces for the UUTs.

As shown, a plurality 710 of optical UUTs 710-1 . . . 710-N (whichincludes at least one optical UUT 710-i) and an alignment referencecomponent 714 are disposed in a spatially-stable and (at least for theduration of the alignment and measurement steps of the process)spatially-invariable relationship with respect to one another. This isachieved by, for example, affixing the optical UUT(s) as well as thealignment reference component in or juxtaposing these elements with asingle, common mounting tray 720

In one non-limiting example, the tray 720 can be formed by reaming anarray of holes openings 724 in a metal plate or substrate whilecontrolling the plate's flatness and positions and dimensioned of themounting holes 723 down to microns. Then a set of surrogate opticalcomponent mounts or holders 508 (as described above) is fabricated thatfit with tight tolerance into the reamed holes 724. The surrogate lensmounts 508 are provided with the fiduciary feature 522 thatappropriately match and/or are congruent with the fiduciary interfacesurfaces 512 of the plurality 710 of UUTs. The individual UUTs 710-i canthen be seated with required precision onto the respective surrogatemounts 508 that, in turn, are placed into the tray 720.

Depending on the specifics of the particular implementation, thealignment reference component 724 may be configured as at least one ofthe CGH 418, a reference mount similar to the mount 508, and thecombination 618 of the surrogate optics and the mount 508.

As was already discussed above, the optical system 428 is generallyequipped with multiple “patches” of the CGH having multiple patterns736-1 . . . 736-N—in other words, the CGH system built into the system428 is configured to provide for a measurement of multiple opticalcomponents at the same time, by transforming an optical wavefrontincident upon it into a plurality of target wavefronts 740-1, . . . ,740-i, 740-N). In addition, the CGH contraption of the optical system428 can include a holographic pattern A that is dedicated for alignment.The position control of the entire tray 720 as a rigid body is carriedout based on the wavesensor measurements, while the alignment of theoptics under test is controlled using the precise datum surfaces.

Accordingly—and referring now specifically to FIG. 7C—the process ofsimultaneous determination of errors in optical wavefronts representingUUT(s) affixed in the tray 720 is initiated by determining the spatialmisalignments of the optical system 428 with respect to the wavesensor(now shown) as discussed, for example, in reference to FIGS. 4A and 6A,followed by the step of aligning the alignment reference component 714,affixed in the tray 720, with respect to an alignment wavefront 734generated at the specific hologram contained in the system 428. (Thisprocess is similar to those discussed in reference to FIGS. 4B, 5B, and6B—depending on the specific contents of the alignment referencecomponent 714, and includes the repositioning/reorientation of the wholetray 720 with all its contents based on the measurements of the errorsof the wavefront 734 reflected at the component 714 through the system428 towards the wavefront sensor.)

A skilled artisan will readily appreciate that—once the component 714fixed in the tray 720 is appropriately aligned—the alignment of theremaining optics (such as the UUT 710-i, for example) with respect tocorresponding local axes of the respective wavefronts from themultiplicity of wavefronts (740-1 . . . 740-I, 740-N) is accomplishedsubstantially automatically, without any additional precautions, and canbe followed by the simultaneous measurements of the UUT(s) in the lightdelivered by the wavefront(s) (740-1, . . . , 740-I, 740-N) in a fashiondiscussed in reference to FIGS. 4C, 5C, 6C, for example.

Notably, an embodiment of the measurement system of the invention can beconfigured for empirical assessment of a wavefront representing a group810 of optical components or systems at once (and containing theinformation not only about a spatial profile of a single surface butalso aggregate information about at least one of the multiple surfacespresent in such group and the distribution of indices of refraction ofthe components of such group). This implementation is schematicallyillustrated in FIG. 8, where the group 810 of individual opticalcomponents is shown to represent a lens system (that includes multipleindividual lens elements) measured in the wavefront 820 formed intransmission from the wavefront 424, incident onto anappropriately-configured holographic patch 824 of the system 428. Whenthe wavefront 820 is reflected at a reference reflector 830 upon havingbeen transmitted through the group 810 and returned—again intransmission through the group 810 and then through the system 428—tothe wavefront sensor 310, such wavefront contains information about thestatus of the lens system 810. In some embodiments, the referencereflector 830 may be include a diffractive optical element.

A skilled person will now appreciate that, in order to carry-out themeasurement discussed in reference to FIGS. 7A-7C and 8, the idea of theinvention is implemented to provide an apparatus that includes awavefront sensor and a first repositionable system containing a set ofholograms (for example, a first alignment hologram, a first measurementhologram, and a second alignment hologram) and disposed across an axisof the apparatus such as i) to form a primary alignment wavefront byreflecting (with one of the alignment holograms) a first portion oflight output from an optical wavefront that has been delivered from thewavefront sensor and that is incident onto the first repositionableoptical system, and (ii) to transmit (through at least one of theremaining holograms) a second portion of such light output. Theapparatus also contains a second repositionable system that includes analignment reference component and at least one optical workpiece held ina fixed position and a fixed orientation with respect to the alignmentreference component. Here, the second repositionable system is disposedto reflect, respectively, a first optical wavefront from the secondportion of the light output and a second optical wavefront from thesecond portion of the light output through the first repositionablesystem back towards the wavefront sensor. The apparatus additionallyincludes a positioner configured to simultaneously change at least oneof a position and an orientation of the alignment reference componentand at least one of a position and an orientation of the at least oneoptical workpiece. Preferably, the apparatus includes a mountingsubstrate installed across the axis and separated from the wavefrontsensor by the first repositionable system. In at least one case, theapparatus is configured to satisfy at least one of the followingconditions: a) the alignment reference component includes at least oneof a reflective hologram attached to the mounting substrate, a referencesurface of an optical element holder attached to the mounting substrate,and a reference surface of an optical element in the optical elementholder attached to the mounting substrate; b) at least one opticalworkpiece is affixed to the mounting substrate; c) at least one opticalworkpiece includes a plurality of optical workpieces, each of which isheld in a fixed position and orientation with respect to the alignmentreference component; d) at least one optical workpiece and the alignmentreference component are removably affixed in the mounting substrate; ande) the positioner is operably cooperated with the mounting substrate tochange at least one of position and orientation of the alignmentreference component and the at least one optical workpiecesimultaneously while and/or during the process of changing at least oneof position and orientation of the mounting substrate. Alternatively orin addition, the apparatus may be configured such that at least one ofthe present optical workpieces is held in an opening defined through themounting substrate, and to include a reference reflector positioned toreceive light from the second wavefront through at least one opticalworkpiece and reflect the so-received light back onto itself, throughthe same optical workpiece, through the first repositionable system, andinto the wavefront sensor.

Such a specific apparatus can be employed measuring an optical wavefrontcharacterizing an optical workpiece as follows. The measuring procedurewould include a step of determining a misalignment of the firstrepositionable system with respect the axis with the use of thesubstantially-spherical optical wavefront incident thereon from awavefront sensor, followed by the step of redirecting first and secondoptical wavefronts (formed by transmitting the substantially-sphericaloptical wavefront respectively through the first and second alignmentholograms) towards the second repositionable system that includes themounting substrate and the alignment reference component disposed in afirst opening of the mounting substrate. The reflected first and secondoptical wavefronts are then formed by interacting the first and secondoptical wavefronts with, respectively, the optical workpiece fixatedlypositioned in a second opening of the mounting substrate and thealignment reference component (which is affixed to the mountingsubstrate), and propagating the reflected first and second opticalwavefronts through the first repositionable system towards the wavefrontsensor. The alignment reference component is then spatially aligned toeliminate at least one of the spatial tilt, azimuthal angular deviation,transverse shift, and longitudinal shift of the alignment referencecomponent with respect to the axis by changing at least one of positionand orientation of the mounting substrate with respect to the axis andbased on a measurement of light (as performed by the optical detectionsystem o the wavefront sensor) from the reflected second opticalwavefront acquired at the wavefront sensor. Finally, a determination ofan error in the reflected first optical wavefront is performed based onan interferometric measurement of light from the first reflected opticalwavefront acquired at the wavefront sensor.

Features of the specific implementation(s) of the idea of the inventionis described with reference to corresponding drawings, in which likenumbers represent the same or similar elements wherever possible. In thedrawings, the depicted structural elements are generally not to scale,and certain components are enlarged relative to the other components forpurposes of emphasis and understanding. It is to be understood that nosingle drawing is intended to support a complete description of allfeatures of the invention. In other words, a given drawing is generallydescriptive of only some, and generally not all, features of theinvention. A given drawing and an associated portion of the disclosurecontaining a description referencing such drawing do not, generally,contain all elements of a particular view or all features that can bepresented is this view, for purposes of simplifying the given drawingand discussion, and to direct the discussion to particular elements thatare featured in this drawing. A skilled artisan will recognize that theinvention may possibly be practiced without one or more of the specificfeatures, elements, components, structures, details, or characteristics,or with the use of other methods, components, materials, and so forth.Therefore, although a particular detail of an embodiment of theinvention may not be necessarily shown in each and every drawingdescribing such embodiment, the presence of this detail in the drawingmay be implied unless the context of the description requires otherwise.In other instances, well known structures, details, materials, oroperations may be not shown in a given drawing or described in detail toavoid obscuring aspects of an embodiment of the invention that are beingdiscussed.

A person of ordinary skill in the art will readily appreciate thatreferences throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. Accordingly—as the skilled artisan will readilyappreciate—while in this specification the embodiments have beendescribed in a way that enables a clear and concise specification to bewritten, it is intended that substantially none of the describedembodiments can be employed only by itself to the exclusion of otherembodiments (to the effect of practically restriction of someembodiments at the expense of other embodiments), and that substantiallyany of the described embodiments may be variously combined or separatedto form different embodiments without parting from the scope of theinvention.

Embodiments of the invention have been described as preferably includinga processor controlled by instructions stored in a memory. The memorymay be random access memory (RAM), read-only memory (ROM), flash memoryor any other memory, or combination thereof, suitable for storingcontrol software or other instructions and data. Those skilled in theart should also readily appreciate that instructions or programsdefining the functions of the present invention may be delivered to aprocessor in many forms, including, but not limited to, informationpermanently stored on non-writable storage media (e.g. read-only memorydevices within a computer, such as ROM, or devices readable by acomputer I/O attachment, such as CD-ROM or DVD disks), informationalterably stored on writable storage media (e.g. floppy disks, removableflash memory and hard drives) or information conveyed to a computerthrough communication media, including wired or wireless computernetworks. In addition, while the invention may be embodied in software,the functions necessary to implement the invention may optionally oralternatively be embodied in part or in whole using firmware and/orhardware components, such as combinatorial logic, Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) orother hardware or some combination of hardware, software and/or firmwarecomponents.

For the purposes of this disclosure and the appended claims, the use ofthe terms “substantially”, “approximately”, “about” and similar terms inreference to a descriptor of a value, element, property orcharacteristic at hand is intended to emphasize that the value, element,property, or characteristic referred to, while not necessarily beingexactly as stated, would nevertheless be considered, for practicalpurposes, as stated by a person of skill in the art. These terms, asapplied to a specified characteristic or quality descriptor means“mostly”, “mainly”, “considerably”, “by and large”, “essentially”, “togreat or significant extent”, “largely but not necessarily wholly thesame” such as to reasonably denote language of approximation anddescribe the specified characteristic or descriptor so that its scopewould be understood by a person of ordinary skill in the art. In onespecific case, the terms “approximately”, “substantially”, and “about”,when used in reference to a numerical value, represent a range of plusor minus 20% with respect to the specified value, more preferably plusor minus 10%, even more preferably plus or minus 5%, most preferablyplus or minus 2% with respect to the specified value. As a non-limitingexample, two values being “substantially equal” to one another impliesthat the difference between the two values may be within the range of+/−20% of the value itself, preferably within the +/−10% range of thevalue itself, more preferably within the range of +/−5% of the valueitself, and even more preferably within the range of +/−2% or less ofthe value itself

The use of these terms in describing a chosen characteristic or conceptneither implies nor provides any basis for indefiniteness and for addinga numerical limitation to the specified characteristic or descriptor. Asunderstood by a skilled artisan, the practical deviation of the exactvalue or characteristic of such value, element, or property from thatstated falls and may vary within a numerical range defined by anexperimental measurement error that is typical when using a measurementmethod accepted in the art for such purposes.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole.

1. An apparatus for measuring an optical wavefront representing anoptical workpiece, the apparatus having an axis and comprising: awavefront sensor; a first repositionable system containing a firstalignment hologram, a first measurement hologram, and a second alignmenthologram and disposed across the axis to form a primary alignmentwavefront by reflecting a first portion of light output from an opticalwavefront delivered from the wavefront sensor and incident onto thefirst alignment hologram, and to transmit a second portion of said lightoutput through the second alignment hologram; and a secondrepositionable system that contains: a) a reflective hologram anddisposed across the axis to reflect light from the second portionthrough the first optical system to form a secondary alignment wavefrontpropagating through the first repositionable system towards thewavefront sensor; and b) tangible fiduciary references outside of thereflective hologram configured to spatially align the workpiece withrespect to said fiduciary references.
 2. An apparatus according to claim1, wherein the wavefront sensor includes an optical interferometer. 3.An apparatus according to claim 2, wherein the first repositionablesystem is disposed at a location at which said optical wavefrontdelivered from the wavefront sensor is either spatially-diverging orspatially converging.
 4. An apparatus according to claim 1, wherein thereflective hologram includes at least one reflective diffraction patternconfigured to form the secondary alignment wavefront that represents atleast one of the spatial tilt, azimuthal angular deviation, transverseshift, and longitudinal shift of the second repositionable system withrespect to the axis.
 5. An apparatus according to claim 1, wherein thefirst measurement hologram is a transmissive hologram configured totransform said incident optical wavefront into an aspheric opticalwavefront.
 6. An apparatus according to claim 1, wherein the firstrepositionable system includes a first stand-alone repositionablecomponent containing said first alignment hologram and a secondstand-alone repositionable component containing said first measurementhologram, wherein the first measurement hologram is configured totransform the incident optical wavefront into an aspheric opticalwavefront.
 7. A method for measuring an optical wavefront characterizingan optical workpiece with a wavefront sensor, the method comprising:determining a misalignment of a first system with respect to an axiswith the use of a substantially-spherical optical wavefront incidentthereon from the wavefront sensor, wherein the first system contains afirst alignment hologram, a first measurement hologram, and a secondalignment hologram; redirecting a first optical wavefront, formed bytransmitting the substantially-spherical optical wavefront through thesecond alignment hologram, towards a second system containing at leastone reflective hologram; forming a reflected first optical wavefront byinteracting the first optical wavefront with the at least one reflectivehologram; aligning the second system with respect to the axis byreducing at least one of spatial tilt, azimuthal angular deviation,transverse shift, and longitudinal shift of the at least one reflectivehologram by measuring an error of the reflected first optical wavefront,that has transmitted through the first optical system, with thewavefront sensor; juxtaposing and affixing the optical workpiece withthe second system in reference to visually-perceivable fiducial featuresof the second system; redirecting a second optical wavefront, formed bytransmitting the substantially-spherical optical wavefront through thefirst alignment hologram, towards the second system to form a reflectedsecond optical wavefront by interacting the second optical wavefrontwith the workpiece; and determining an error in the reflected secondoptical wavefront based on a measurement of light from the reflectedsecond optical wavefront acquired at the wavefront sensor.
 8. A methodaccording to claim 7, wherein said determining the misalignment of thefirst system includes reflecting the substantially-spherical wavefrontat the first alignment hologram contained in a first component of thefirst system, and wherein said redirecting the second optical wavefrontincludes transmitting the substantially—spherical wavefront though oneof a) the first measurement hologram contained in a second component ofthe first system, said first and second components of the first systembeing spatially distinct from one another; and b) the first measurementhologram contained in the same first component of the first system.
 9. Amethod according to claim 7, wherein said determining the error includesat least one of i) reflecting the second optical wavefront at a surfaceof the optical workpiece; and ii) transmitting the second opticalwavefront through the optical workpiece twice.
 10. A method according toclaim 9, comprising reflecting light from the second optical wavefrontat a reference reflector separated from the first system by the opticalworkpiece.
 11. A method according to claim 7, further comprising:partially reflecting the substantially-spherical optical wavefront atthe first alignment hologram of the first system towards the wavefrontsensor to form a reflected wavefront, at least reducing the misalignmentof the first system by minimizing a figure of merit determined based ona measurement of a phase of the reflected wavefront at the wavefrontsensor; and spatially fixating a component of the first system withrespect to the axis after said figured of merit has been minimized. 12.A method according to claim 7, wherein said aligning the second systemincludes reducing each of spatial tilt, azimuthal angular deviation,transverse shift, and longitudinal shift of the at least one reflectivehologram by measuring said error of the reflected first opticalwavefront, that has transmitted through the first optical system, withwavefront sensor.
 13. A method for measuring an optical wavefrontcharacterizing an optical workpiece, the method comprising: determininga misalignment of a first system with respect an axis with the use of asubstantially-spherical optical wavefront incident thereon from awavefront sensor, wherein the first system contains at least a firstalignment hologram, a first measurement hologram, and a second alignmenthologram; redirecting first and second optical wavefronts, formed bytransmitting the substantially-spherical optical wavefront respectivelythrough the first and second alignment holograms, towards a secondsystem containing a mounting substrate and an alignment referencecomponent disposed in a first opening of the mounting substrate; formingreflected first and second optical wavefronts by respectivelyinteracting the first and second optical wavefronts with the opticalworkpiece fixatedly positioned in a second opening of the mountingsubstrate and the alignment reference component; propagating thereflected first and second optical wavefronts through the first systemtowards the wavefront sensor; spatially aligning the alignment referencecomponent affixed at the mounting substrate to eliminate at least one ofthe spatial tilt, azimuthal angular deviation, transverse shift, andlongitudinal shift of the alignment reference component with respect tothe axis by changing at least one of position and orientation of themounting substrate with respect to the axis based on a measurement oflight from the reflected second optical wavefront acquired at thewavefront sensor, and determining an error in the reflected firstoptical wavefront based on a measurement of light from the firstreflected optical wavefront acquired at the wavefront sensor.
 14. Amethod according to claim 13, comprising substantially nullifying themisalignment of the first optical system by: partially reflecting thesubstantially-spherical optical wavefront at the first alignmenthologram of the first system towards the wavefront sensor to form areflected wavefront, at least reducing the misalignment of the firstsystem by minimizing a figure of merit determined based on a measurementof a phase of the reflected wavefront at the wavefront sensor; andspatially fixating a component of the first optical system with respectto the axis after said figured of merit has been minimized.
 15. A methodaccording to claim 14, wherein said determining the error in thereflected first optical wavefront is carried out after the step of atleast reducing the misalignment of the first system has beenaccomplished.
 16. A method according to claim 13, wherein, when thewavefront sensor is an optical interferometer, at least one of thefollowing conditions is satisfied: a) said determining the error in thereflected first optical wavefront includes determining a differencebetween a spatial profile of the reflected first optical wavefront thathas transmitted through the first system towards the interferometer anda spatial profile of a reference optical wavefront generated internallyto the interferometer; b) said spatially aligning the alignmentreference component includes forming alignment optical interferencefringes by optically interfering, at an output of the opticalinterferometer, the reflected second optical wavefront that hastransmitted through the first system and the reference opticalwavefront; and c) said at least reducing the misalignment includesreorienting the component of the first system to substantially eliminateat least one of the spatial tilt, azimuthal angular deviation,transverse shift, and longitudinal shift of said component with respectto the axis based on transformation of the optical interference fringesformed at the output of the optical interferometer.
 17. A methodaccording to claim 13, wherein the first system includes first andsecond spatially-distinct and separable from one another components,wherein the first component contains one or more of the first alignmenthologram, the second alignment hologram, and the first measurementhologram while the second component contains remaining of the firstalignment hologram, the second alignment hologram, and the firstmeasurement hologram.
 18. A method according to claim 13, wherein eachof said steps of redirecting and spatially aligning is carried outwithout a relative movement between the workpiece and the mountingsubstrate and without a relative movement between the alignmentreference component and the mounting substrate.
 19. A method accordingto claim 13, wherein said redirecting the second optical wavefront byinteracting said second optical wavefront with the alignment referencecomponent includes at least one of a) reflecting said second opticalwavefront by a reflective hologram contained in the second opticalsystem; b) reflecting said second optical wavefront at a referencesurface of an optical element holder held in arespectively-corresponding opening of the mounting substrate; and c)reflecting said second optical wavefront at a reference surface of anoptical element mounted in the optical element holder held in therespectively-corresponding opening of the mounting substrate.
 20. Amethod according to claim 13, wherein the redirecting first and secondoptical wavefronts includes: redirecting a plurality of measurementoptical wavefronts, each formed by transmitting thesubstantially-spherical optical wavefront through a plurality of themeasurement holograms of the first system, and forming simultaneously aplurality of reflected measurement optical wavefronts by respectivelyinteracting each of the plurality of measurement optical wavefronts witha corresponding workpiece from a plurality of workpieces disposed inrespectively-corresponding openings in the mounting substrate.
 21. Anapparatus for measuring an optical wavefront representing an opticalworkpiece, the apparatus having an axis and comprising: a wavefrontsensor; a first repositionable system containing a first alignmenthologram, a first measurement hologram, and a second alignment hologramand disposed across the axis to form a primary alignment wavefront byreflecting a first portion of light output from an optical wavefrontdelivered from the wavefront sensor and incident onto the firstrepositionable optical system, and to transmit a second portion of saidlight output; a second repositionable system containing an alignmentreference component and at least one optical workpiece held in a fixedposition and a fixed orientation with respect to the alignment referencecomponent, wherein the second repositionable system is disposed toreflect, respectively, a first optical wavefront from the second portionand a second optical wavefront from the second portion through the firstrepositionable system towards the wavefront sensor; and a positionerconfigured to simultaneously change at least one of a position and anorientation of the alignment reference component and at least one of aposition and an orientation of the at least one optical workpiece. 22.An apparatus according to claim 21, comprising a mounting substrateinstalled across the axis and separated from the wavefront sensor by thefirst repositionable system, and wherein at least one of the followingconditions is satisfied: a) the alignment reference component includesat least one of a reflective hologram attached to the mountingsubstrate, a reference surface of an optical element holder attached tothe mounting substrate, and a reference surface of an optical element inthe optical element holder attached to the mounting substrate; b) the atleast one optical workpiece is affixed to the mounting substrate; c) theat least one optical workpiece includes a plurality of opticalworkpieces, each held in a fixed position and orientation with respectto the alignment reference component; d) the at least one opticalworkpiece and the alignment reference component are removably affixed inthe mounting substrate; and e) the positioner is operably cooperatedwith the mounting substrate to change the at least one of position andorientation of the alignment reference component and the at least oneoptical workpiece simultaneously while changing at least one of positionand orientation of the mounting substrate.
 23. An apparatus according toclaim 21, wherein the at least one optical workpiece is held in anopening defined through the mounting substrate, and further comprising areference reflector positioned to receive light from the secondwavefront through the at least one optical workpiece and reflect saidlight back onto itself, through the at least one optical workpiece, thefirst repositionable system, and into the wavefront sensor.